The present invention relates to internal film stress evaluation. More particularly, the present invention relates to internal film stress evaluation using revolving ellipsometry at high lateral resolution.
Today's semiconductor fabrication technology typically employs more than one layer of film (e.g., multi-layer systems) to put more features into integrated circuit (IC) chips. In general, films in these multi-layer systems induce internal film stress due to lattice mismatch at the interfaces of such layers. In addition, different mechanical material properties such as elasticity, thermal expansion and the like also contribute to the internal film stress. It should be appreciated that the term "film" as used herein refers to a material having a smooth or planar surface. The film may be comprised of a uniform material or mixed materials in the shape of, for example, a substrate, a membrane, a layer, a stratum, a sheet, or any form that includes a smooth or planar surface. A sample or system may be comprised of one or more films and are used interchangeably herein.
Stress in a film is undesirable because the stress often leads to serious degradation of the material properties of the film. For example, a stressed multi-layer system may lose its intended electrical characteristics. Further, stress may induce dislocations at the top of deep trenches. In a memory chip such as dynamic random-access-memory (DRAM), for instance, the dislocations can cause variable retention time of electrical signals. Accordingly, accurate evaluation of stress is crucial in order to minimize and understand the effect of internal stress.
In the past, conventional techniques typically determined internal film stress only on bendable materials. In particular, the conventional techniques measured the bending or curvature of a macroscopic uniform sample (e.g., a wafer, a substrate, etc.) using optical (e.g., beam deflection) or electrical (e.g., capacity) techniques. For example, a beam of light is directed on the bendable sample and a curvature radius between the angles of incidence and reflection on the sample is measured. Then, a film is deposited on the bendable sample. The deposited film induces the sample to bend. A beam of light is then directed on the bent sample and a curvature radius between the angles of incidence and reflection on the bent sample is measured. The difference between the radius obtained before the layer deposition and the radius after the deposition is then computed to obtain a difference term. The stress is then determined by correlating the difference term using the well known Stoney's formula by applying known elastic constants of the sample material.
Unfortunately, these conventional techniques have several drawbacks that limit their application. For example, the conventional approaches cannot be used on unbendable samples or solid samples that are too massive to show any bending. Further, these techniques require macroscopic uniform samples. In addition, since these techniques are designed for macroscopic uniform samples, they cannot be applied at high lateral resolutions necessary for analyzing microscopic features. For example, patterned materials typically exhibit a sensitive and microscopic geometry. In a patterned material, stress gradients occur at internal edges and vertical sidewalls of trench structures. Stress peaks typically occur at an edge of a trench and may cause a dislocation.
In view of the foregoing, what is desired is a method and apparatus for evaluating internal film stress on any samples including unbendable or solid samples at high lateral resolution.